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The pressure of the gas in the container can be measured, indicating exactly how much product (fuel) is still remaining.

Many propellants liquefy under pressure. As long as any propellant remains in liquid form, the pressure will be a function of pressure, rather than of the quantity remaining. This allows things like butane lighters to be constructed very cheaply, since the pressure when nearly full is no greater than the pressure when nearly empty.

As long as any propellant remains in liquid form, the pressure will be a function of pressure

That seems a pretty safe bet. ;-)

You meant to write, "a function of temperature", of course, but I couldn't resist joking about the inadvertent tautology.

I am assuming of course, that NASA knows exactly which fuels they are using in their spacecraft, and not guessing or filling them up with random, various fuels that act differently when under pressure..
The characteristics of those fuels should be well known to the engineers ( and rocket scientists ) that designed and built them..

I can't imagine an explosives expert not knowing the properties of his explosives, the burn rate of his fuses, etc..
All of those calculations are supposedly done before the launch date is ever approved..
Weight being a primary consideration, only the necessary amount of fuel for the mission is put on board..
It's designed that way..
Telemetry should be able to tell how much fuel is available..

Likewise, I can't imagine sending a multi-million dollar spacecraft into orbit without knowing exactly how much fuel it has, the burn rate, etc..

My cheese-whiz analogy may not be cost effective, or even scientific, but it points out just one example off the top of my head that could be used to account for fuel usage and remaining capacity..
Please note, the pressurized gas in this instance is not used as a propellant, it only provides pressure for the expulsion of the cheeze whiz.. It's pressure would indeed, remain constant, it never leaves the container..
Sensors could read the positions of microchip tags on the fuel bag to determine remaining volume..
Also, fuel consumption could be determined by measuring the flow of the fuel (or gas) to the engine..
I'm not a scientist, so I may be wrong, but it would seem that there must be some way of determining the ratio of liquid to gas (vapor pressure or boiling point) that would indicate the amount of fuel left in a fuel tank..

Just sayin'...
There must be a dozen ways to determine how much fuel is left, how much has been used, etc..